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Old 08/12/18, 07:38 PM   #1
ECStein
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302 Hesitation and Backfire at 4k

I have a 72 with a 302 and Edelbrock 1404 carb. Vehicle idles and drives fine. If I dig in at WOT in 1st or go to WOT in 2nd the engines sputters, feels like its losing power, and backfires out of the exhaust.

I've checked timing (10 BTC), vacuum (18 @ idle) and fuel pressure (4 at idle). The problem persists - it always happens at 3.8k to 4k and causes the needle on the tach to bounce.

Any thoughts?
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Old 08/13/18, 09:08 AM   #2
B RON CO
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Hi, I would check the ignition system. If the tune up parts are old I would probably replace them. If the wires are arcing ( look under the hood at night), or the points are corroded, or the cap has cracks or carbon tracking that could cause your problem.
You could unhook the tach to see if it is causing the problem.
Good luck
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Old 08/13/18, 09:34 AM   #3
Broncobowsher
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Bouncing tach is a big clue. The tach reads ignition speed. Doesn't care about fuel. You could be out of gas coasting down a hill and the ignition will still fire away and the tach will still read correctly.

Since the tach goes funky at the same time the engine runs bad, that tell me that you have an ignition issue. What parts are in your ignition system?

"She's built like a steakhouse but she handles like a bistro"-Zapp Brannigan
Why do people keep expecting "normal" from me?
Full throttle, it either solves the problem or ends the suspense.
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Old 08/13/18, 10:04 AM   #4
bax
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You are lean. The backfire at high RPM is a lean backfire. Not ignition in my opinion. Check your float levels, your pump PSI and your fuel filters.

1971 U15 Ol'blue 5.0/4r70w/205/ 4,11. 1968 U15 - EFI 408C/C6/205/4.88/37's/hi9/ARB's/hydro-boost/full cage/4wdisk/PS/powerasswipe/bla-bla-bla!
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Old 08/13/18, 11:40 PM   #5
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If you check your fuel mixture and you're not running lean, check the total timing of your distributor, as you might not getting enough timing and need to adjust your distributor. 10˚ is a good base timing to get your engine going, but in my experience, it rarely stays at 10˚.
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Old 08/14/18, 07:18 AM   #6
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Does the tach bounce match what the engine is actually doing? IF it is dropping 1000 RPM and back in under a second type of thing that the engine can't actually be doing. That shows electrical issues. Think of the symptoms if you were driving and flicked the key off and back on. The tach will bounce around (even though engine speed doesn't really change) and you get backfires through the exhaust. Exactly what you describe.

"She's built like a steakhouse but she handles like a bistro"-Zapp Brannigan
Why do people keep expecting "normal" from me?
Full throttle, it either solves the problem or ends the suspense.
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